Modbus protocol for SCADA in SCADA systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the Modbus function code for reading coils.

SCADA systems

modbus_request = {"function_code": [1], "address": 0x0013, "quantity": 19}

Drag options to blanks, or click blank then click option'

A0x01

B0x03

C0x10

D0x05

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x03 instead of 0x01 for reading coils.

The Modbus function code 0x01 is used to read coils (digital outputs).

2fill in blank

medium

Complete the code to specify the Modbus register type for holding registers.

SCADA systems

register_type = "[1]"

Drag options to blanks, or click blank then click option'

Acoil

Bholding_register

Cinput_register

Ddiscrete_input

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing input_register with holding_register.

Holding registers are used to store data that can be read or written. The correct type is holding_register.

3fill in blank

hard

Fix the error in the Modbus RTU frame checksum calculation.

SCADA systems

checksum = calculate_crc([1])

Drag options to blanks, or click blank then click option'

Amodbus_frame

Bmodbus_frame[-2:]

Cmodbus_frame[2:]

Dmodbus_frame[:-2]

Attempts:

3 left

💡 Hint

Common Mistakes

Including checksum bytes in calculation causing errors.

The checksum is calculated over the entire frame except the last two bytes which contain the checksum itself. So use modbus_frame[:-2].

4fill in blank

hard

Fill both blanks to create a Modbus TCP request header with transaction ID and protocol ID.

SCADA systems

header = [1].to_bytes(2, 'big') + [2].to_bytes(2, 'big') + b'\x00\x06'

Drag options to blanks, or click blank then click option'

Atransaction_id

Bprotocol_id

Cunit_id

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping protocol_id and transaction_id.

The Modbus TCP header starts with a 2-byte transaction ID and a 2-byte protocol ID. So the first blank is transaction_id and the second is protocol_id.

5fill in blank

hard

Fill all three blanks to build a dictionary comprehension filtering coils with status ON.

SCADA systems

active_coils = {addr: status for addr, status in coils.items() if status [1] [2] and addr [3] 100}

Drag options to blanks, or click blank then click option'

A==

BTrue

C<

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using != True or wrong comparison operators.

The comprehension filters coils where status is True (ON) and address is less than 100. So the blanks are ==, True, and <.

Practice

1. What is the primary purpose of the Modbus protocol in SCADA systems?

easy

A. To store large amounts of historical data

B. To provide a graphical user interface for SCADA

C. To encrypt data transmissions for security

D. To enable communication between SCADA and industrial devices

Solution

1. Step 1: Understand Modbus role in SCADA

   Modbus is designed to allow SCADA systems to communicate with devices like sensors and controllers.

2. Step 2: Identify the main function

   It is not for UI, data storage, or encryption but for communication.

3. Final Answer:

   To enable communication between SCADA and industrial devices -> Option D

4. Quick Check:

   Modbus = communication protocol [OK]

Hint: Modbus connects SCADA to devices, not UI or storage [OK]

Common Mistakes:

• Confusing Modbus with data storage systems
• Thinking Modbus provides user interfaces
• Assuming Modbus encrypts data by default

2. Which of the following is the correct Modbus function code to read holding registers?

easy

A. 0x03

B. 0x06

C. 0x10

D. 0x01

Solution

1. Step 1: Recall Modbus function codes

   Function code 0x03 is used to read holding registers in Modbus.

2. Step 2: Verify other codes

   0x06 is for writing a single register, 0x10 for multiple registers, 0x01 for reading coils.

3. Final Answer:

   0x03 -> Option A

4. Quick Check:

   Read holding registers = 0x03 [OK]

Hint: 0x03 always reads holding registers in Modbus [OK]

Common Mistakes:

• Mixing read and write function codes
• Confusing coil read (0x01) with register read
• Using write codes for reading operations

3. Given the following Modbus request frame in hexadecimal: 01 03 00 0A 00 02 C5 CD, what does it represent?

medium

A. Write 2 registers starting at address 10 to device 1

B. Read 2 holding registers starting at address 10 from device 1

C. Read 2 coils starting at address 10 from device 1

D. Read 10 holding registers starting at address 2 from device 1

Solution

1. Step 1: Decode the Modbus frame parts

   01 = device address, 03 = read holding registers, 00 0A = start address 10, 00 02 = read 2 registers.

2. Step 2: Confirm the meaning

   This frame requests reading 2 holding registers starting at address 10 from device 1.

3. Final Answer:

   Read 2 holding registers starting at address 10 from device 1 -> Option B

4. Quick Check:

   Function 03 + address 10 + count 2 = Read 2 holding registers starting at address 10 from device 1 [OK]

Hint: Function code 03 + count 2 means read 2 registers [OK]

Common Mistakes:

• Misreading address and count bytes
• Confusing read and write function codes
• Mixing coil and register addresses

4. A Modbus RTU message is sent but the SCADA system reports a CRC error. What is the most likely cause?

medium

A. Using function code 0x10 instead of 0x03

B. Wrong device address used in the message

C. Incorrect calculation of the CRC checksum

D. Message length is too short

Solution

1. Step 1: Understand CRC role in Modbus RTU

   CRC checksum verifies message integrity; an error means checksum mismatch.

2. Step 2: Identify common CRC error cause

   Incorrect CRC calculation or corruption causes CRC errors, not device address or function code choice.

3. Final Answer:

   Incorrect calculation of the CRC checksum -> Option C

4. Quick Check:

   CRC error = bad checksum calculation [OK]

Hint: CRC errors usually mean checksum miscalculation [OK]

Common Mistakes:

• Assuming wrong device address causes CRC error
• Confusing function code errors with CRC errors
• Ignoring message corruption possibilities

5. You need to configure a SCADA system to write multiple registers starting at address 20 on device 5 using Modbus. Which function code and message structure should you use?

hard

A. Function code 0x10 with device 5 address, start address 20, number of registers, and data bytes

B. Function code 0x06 with device 5 address and single register data

C. Function code 0x03 with device 5 address and read parameters

D. Function code 0x01 with device 5 address and coil data

Solution

1. Step 1: Identify correct function code for writing multiple registers

   Function code 0x10 is used to write multiple holding registers in Modbus.

2. Step 2: Confirm message structure

   The message must include device address (5), start address (20), number of registers, and data bytes to write.

3. Final Answer:

   Function code 0x10 with device 5 address, start address 20, number of registers, and data bytes -> Option A

4. Quick Check:

   Write multiple registers = 0x10 + address + data [OK]

Hint: Use 0x10 for multiple register writes, not 0x06 or 0x03 [OK]

Common Mistakes:

• Using 0x06 which writes only one register
• Confusing read (0x03) with write operations
• Using coil function codes for register writes